blender to unity error
Based on what I see in the image you posted and as you said yourself, the main problem is the fact that your model has N-gons, which is a polygon made of 5 or more vertices, in your case that is the top face. Unity actually struggles with N-gons, so it is best to make the model with only Quads and/or Triangles. Try deleting the top face and make two faces like the ones you have on the front of your model. To make sure this is actually the problem, check if the black dot on the top face is actually at the center of that face, if not you definitely have an N-gon there. Otherwise (=if the black dot is in the center of the face) the problem could also be that the middle vertex on the top (red circle in the image below) might not be attached to the top face. Either way the ideal solution should be creating two top faces like the one I highlighted in the image below.